Brake master cylinders are well known in the brake art for converting a mechanical force that is input from a brake pedal into a fluid pressure acting on a brake actuator at a vehicle wheel to effect vehicle braking. Power assisted brakes have a power booster attached to the master cylinder to reduce the amount of brake pedal input force required from a driver.
One method for supplying master cylinders involves a brake component supplier assembling a master cylinder to a power booster and then shipping the completed subassembly to a vehicle manufacturer's plant for final assembly into a vehicle.
A second method involves separately supplying a master cylinder to a vehicle manufacturer's plant because a power booster has already been installed into a vehicle. In contrast with the first method, the master cylinder is assembled into the power booster at the vehicle manufacturer's plant, rather than the supplier's plant. As a result, a portion of the master cylinder which extends into the power booster is now exposed to foreign material and foreign objects during shipping from the brake component supplier and during storage, retrieval and installation at the vehicle manufacturer's plant prior to assembly within a booster. Part of the exposed portion includes a reciprocating member having an outer surface which engages a seal. Damage to the outer surface by foreign material or foreign objects can result in seal failure and loss of master cylinder fluid which could ultimately result in brake failure.
Attempts have been made to ease assembly and protect the exposed reciprocating member of master cylinders during shipment and later during assembly at the vehicle manufacturer's plant. A known design includes a single seal having two diameter sections separated by a shoulder. One end of the seal is attached to a front face of a master cylinder housing for sealing the interface between the housing and a brake booster. The other end of the seal is attached to a piston and aids in aligning the piston with a push rod in the booster. Another known design includes a funnel shaped centering guide attached to the end of a piston rod and also has a protective sleeve with one end sealingly attached to the housing and the other end being open. In this design, the protective sleeve is a rigid component.
The previous attempts at protecting a reciprocating member of a master cylinder have not been completely effective. The first mentioned design attempts to provide a seal that aids in aligning the piston to a push rod, protects the piston surface, and also seals the housing to booster interface. Such a seal design is not desirable since it does not allow for easy assembly of the push rod to the piston and it includes a larger diameter seal portion which is more susceptible to contact with a booster opening and therefore, more susceptible to damage. The other design does not effectively seal out foreign material or objects. The open end of the sleeve allows debris and tools to reach a sealing surface on the piston rod leading to greater chances of seal failure.